1. Field of the Invention
The present invention relates generally to wireless communication systems and, more specifically, to transmission and reception of physical downlink control channels.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from transmission points such as Base Stations (BSs) or NodeBs to User Equipments (UEs) and an UpLink (UL) that conveys transmission signals from UEs to reception points such as NodeBs. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a cellular phone, a personal computer device, etc. A NodeB, which is generally a fixed station, may also be referred to as an access point or other equivalent terminology.
DL signals includes data signals, which carry information content, control signals, and Reference Signals (RS), which are also known as pilot signals. A NodeB conveys data information to UEs through respective Physical Downlink Shared CHannels (PDSCHs) and control information through respective Physical Downlink Control CHannels (PDCCHs). UL signals also include data signals, control signals and RS. UEs convey data information to NodeBs through respective Physical Uplink Shared CHannels (PUSCHs) and control information through respective Physical Uplink Control CHannels (PUCCHs). A UE transmitting data information may also convey control information through a PUSCH.
Downlink Control Information (DCI) serves several purposes and is conveyed through DCI formats in respective PDCCHs. For example, DCI includes DL Scheduling Assignments (SAs) for PDSCH receptions and UL SAs for PUSCH transmissions. As PDCCHs are a major part of a total DL overhead, the required resources required to transmit PDCCHs directly reduce DL throughput. One method for reducing PDCCH overhead is to scale its size according to the resources required to transmit the DCI formats during a DL Transmission Time Interval (TTI). When Orthogonal Frequency Division Multiple (OFDM) is used as a DL transmission method, a Control Channel Format Indicator (CCFI) parameter transmitted through a Physical Control Format Indicator CHannel (PCFICH) can be used to indicate a number of OFDM symbols occupied by PDCCHs in a DL TTI.
FIG. 1 is a diagram illustrating a conventional structure for PDCCH transmissions in a DL TTI.
Referring to FIG. 1, a DL TTI includes one subframe having N=14 OFDM symbols. A DL control region that includes PDCCH transmissions occupies a first M OFDM symbols 110. A remaining N-M OFDM symbols are used primarily for PDSCH transmissions 120. A PCFICH 130 is transmitted in some sub-carriers, also referred to as Resource Elements (REs), of a first OFDM symbol and includes 2 bits indicating a DL control region size of M=1, or M=2, or M=3 OFDM symbols. Moreover, some OFDM symbols also contain respective RS REs, 140 and 150. These RS are transmitted substantially over an entire DL operating BandWidth (BW) and are referred to as Common RS (CRS), as they can be used by each UE to obtain a channel estimate for its DL channel medium and to perform other measurements. The BW unit for a PDSCH or a PUSCH over a subframe is referred to as a Physical Resource Block (PRB). A PRB includes several REs, such as for example 12 REs.
Additional control channels may be transmitted in a DL control region, but are not shown for brevity. For example, when using a Hybrid Automatic Repeat reQuest (HARQ) process for data transmission in a PUSCH, a NodeB may transmit HARQ-ACKnowledgement (ACK) information in a Physical Hybrid-HARQ Indicator CHannel (PHICH) to indicate to a UE whether its previous transmission of each data Transport Block (TB) in a PUSCH was correctly received (i.e., through an ACK) or incorrectly received (i.e., through a Negative ACK (NACK)).
In addition to the CRS in FIG. 1, other DL RS types are the DeModulation RS (DMRS), which may only be transmitted in PRBs used for a PDSCH transmission.
FIG. 2 is a diagram illustrating a conventional DMRS structure.
Referring to FIG. 2, DMRS REs 210 and 215 in a PRB convey DMRS from four Antenna Ports (APs). A DMRS transmission from a first AP applies an Orthogonal Covering Code (OCC) of {1, 1} 220 over two DMRS REs located in a same frequency position and are successive in the time domain, while a second AP applies an OCC of {1, −1} 225. A DMRS transmission from a third AP is in different REs than DMRS transmissions from a first AP, and the third AP applies an OCC of {1, 1} 230 over two DMRS REs located in a same frequency position and are successive in the time domain, while a fourth AP applies an OCC of {1, −1} 235. A UE receiver can estimate a channel experienced by a signal from an AP by removing a respective OCC at respective DMRS Res, and may also possibly estimate the channel by interpolating across respective DMRS REs in a subframe.
FIG. 3 is a diagram illustrating a conventional encoding process for a DCI format.
Referring to FIG. 3, a NodeB separately codes and transmits each DCI format in a respective PDCCH. A Radio Network Temporary Identifier (RNTI) for a UE, for which a DCI format is intended for, masks a Cyclic Redundancy Check (CRC) of a DCI format codeword in order to enable the UE to identify that a particular DCI format is intended for the UE. The CRC of (non-coded) DCI format bits 310 is computed using a CRC computation operation 320, and the CRC is then masked using an exclusive OR (XOR) operation 330 between CRC and RNTI bits 340. The XOR operation 330 is defined as: XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. The masked CRC bits are appended to DCI format information bits using a CRC append operation 350, channel coding is performed using a channel coding operation 360 (e.g. an operation using a convolutional code), followed by rate matching operation 370 applied to allocated resources, and finally, an interleaving and modulation 380 operation is performed, and the output control signal 390 is transmitted. In the present example, both a CRC and a RNTI include 16 bits.
FIG. 4 is a diagram illustrating a conventional decoding process for a DCI format.
Referring to FIG. 4, a UE receiver performs the reverse operations of a NodeB transmitter to determine whether the UE has a DCI format assignment in a DL subframe. A received control signal 410 is demodulated and the resulting bits are de-interleaved at operation 420, a rate matching applied at a NodeB transmitter is restored through operation 430, and data is subsequently decoded at operation 440. After decoding the data, DCI format information bits 460 are obtained after extracting CRC bits 450, which are then de-masked 470 by applying the XOR operation with a UE RNTI 480. Finally, a UE performs a CRC test 490. If the CRC test passes, a UE determines that a DCI format corresponding to the received control signal 410 is valid and determines parameters for signal reception or signal transmission. If the CRC test does not pass, a UE disregards the presumed DCI format.
To avoid a PDCCH transmission to a UE that is blocking a PDCCH transmission to another UE, a location of each PDCCH in the time-frequency domain of a DL control region is not unique. Therefore, a UE must perform multiple decoding operations to determine whether there are PDCCHs intended for the UE in a DL subframe. The REs carrying a PDCCH are grouped into Control Channel Elements (CCEs) in the logical domain. For a given number of DCI format bits in FIG. 2, a number of CCEs for a respective PDCCH depends on a channel coding rate (in the present example, a Quadrature Phase Shift Keying (QPSK) is used as the modulation scheme). A NodeB may use a lower channel coding rate (i.e., more CCEs) for transmitting PDCCHs to UEs experiencing a low DL Signal-to-Interference and Noise Ratio (SINR) than to UEs experiencing a high DL SINR. The CCE aggregation levels may include, for example, 1, 2, 4, and 8 CCEs.
For a PDCCH decoding process, a UE may determine a search space for candidate PDCCHs after the UE restores the CCEs in the logical domain according to a common set of CCEs for all UEs (i.e., a Common Search Space (CSS)) and according to a UE-dedicated set of CCEs (i.e., a UE-Dedicated Search Space (UE-DSS)). A CSS may include the first C CCEs in the logical domain. A UE-DSS may be determined according to a pseudo-random function having UE-common parameters as inputs, such as the subframe number or the total number of CCEs in the subframe, and UE-specific parameters such as the RNTI. For example, for CCE aggregation levels Lε{1,2,4,8}, the CCEs corresponding to PDCCH candidate in are given by Equation (1).CCEs for PDCCH candidate m=L·{(Yk+m)mod └NCCE,k/L┘}+i   ( )
In Equation (1), NCCE,k is the total number of CCEs in subframe k, i=0, . . . , L−1, m=0, . . . , M(L)−1, and M(L) is the number of PDCCH candidates to monitor in the search space. Exemplary values of M(L) for Lε{1,2,4,8} are, respectively, {6, 6, 2, 2}. For the UE-CSS, Yk=0. For the UE-DSS, Yk=(A·Yk-1)mod D where Y1=RNTI≠0, A=39827 and D=65537.
DCI formats conveying information to multiple UEs are transmitted in a CSS. Additionally, if enough CCEs remain after the transmission of DCI formats conveying information to multiple UEs, a CSS may also convey some UE-specific DCI formats for DL SAs or UL SAs. A UE-DSS exclusively conveys UE-specific DCI formats for DL SAs or UL SAs. For example, a UE-CSS may include 16 CCEs and support 2 DCI formats with L=8 CCEs, or 4 DCI formats with L=4 CCEs, or 1 DCI format with L=8 CCEs and 2 DCI formats with L=4 CCEs. The CCEs for a CSS are placed first in the logical domain (prior to interleaving).
FIG. 5 is a diagram illustrating a conventional transmission process of a DCI format in a respective PDCCH.
Referring to FIG. 5, after channel coding and rate matching is performed (as described with reference to FIG. 3), encoded DCI format bits are mapped, in the logical domain, to CCEs of a PDCCH. The first 4 CCEs (L=4), CCE1 501, CCE2 502, CCE3 503, and CCE4 504 are used for PDCCH transmission to UE1. The next 2 CCEs (L=2), CCE5 511 and CCE6 512, are used for PDCCH transmission to UE2. The next 2 CCEs (L=2), CCE7 521 and CCE8 522, are used for PDCCH transmission to UE3. Finally, the last CCE (L=1), CCE9 531, is used for PDCCH transmission to UE4.
The DCI format bits are then scrambled, at step 540, by a binary scrambling code, and the scrambled bits are modulated at step 550. Each CCE is further divided into Resource Element Groups (REGs). For example, a CCE including 36 REs can be divided into 9 REGs that each include 4 REs. In step 560, interleaving is applied among REGs in blocks of four QPSK symbols. For example, a block interleaver may be used where interleaving is performed on symbol-quadruplets (i.e., four QPSK symbols corresponding to the four REs of a REG) instead of on individual bits. After interleaving the REGs, in step 570, a resulting series of QPSK symbols may be shifted by J symbols, and finally, in step 580, each QPSK symbol is mapped to an RE in a DL control region. Therefore, in addition to RSs from NodeB transmitter antennas 591 and 592, and other control channels, such as a PCFICH 593 and a PHICH (not shown), REs in a DL control region contain QPSK symbols for PDCCHs corresponding to DCI formats for UE1 594, UE2 595, UE3 596, and UE4 597.
The control region for transmissions of PDCCHs in FIG. 5 uses a maximum of M=3 OFDM symbols and transmits a control signal substantially over a total operating DL BW. As a consequence, the control region has limited a capacity and cannot achieve interference coordination in the frequency domain. There are several cases where expanded capacity or interference coordination in the frequency domain is needed for transmission of control signals. One such case is a communication system with cell aggregation where the DL SAs or UL SAs to UEs in multiple cells are transmitted in a single cell (for example, because other cells may convey only PDSCH). Another case is extensive use of multi-UE spatial multiplexing of PDSCHs where multiple DL SAs correspond to same PDSCH resources. Another case is when DL transmissions from a first NodeB experience strong interference from DL transmissions from a second NodeB and DL interference co-ordination in the frequency domain between the two NodeBs is needed.
Due to REG-based transmission and interleaving of PDCCHs, the control region cannot be expanded to include more OFDM symbols while maintaining compatible operation with existing UEs that cannot be aware of such expansion. An alternative is to extend the control region in the PDSCH region and use individual PRBs for transmitting new PDCCHs, which will be referred to as Enhanced PDCCHs (E-PDCCHs). A NodeB may configure a UE to perform decoding operations for either or both of PDCCH and E-PDCCHs. Typically, a NodeB configures to a UE a functionality by higher layer signaling such as Radio Resource Control (RRC) signaling.
FIG. 6 is a diagram illustrating a conventional E-PDCCH transmission structure.
Referring to FIG. 6, although E-PDCCH transmissions start immediately after PDCCH transmissions 610 and are transmitted over all remaining DL subframe symbols, the E-PDCCH transmissions may instead start at a predetermined subframe symbol and extend over a part of remaining DL subframe symbols. E-PDCCH transmissions may occur in four PRBs, 620, 630, 640, and 650, while remaining PRBs 660, 662, 664, 666, 668 may be used for PDSCH transmissions. As an E-PDCCH transmission over a given number of subframe symbols may require fewer REs than the number of subframe symbols available in a PRB, multiple E-PDCCHs may be multiplexed in a same PRB. The multiplexing can be in any combination of possible domains (i.e., time domain, frequency domain, or spatial domain) and, in a manner similar to a PDCCH, an E-PDCCH includes at least one Enhanced CCE (E-CCE).
An E-PDCCH transmission may be in a single PRB if a NodeB has accurate information for a channel experienced by a respective UE and can perform Frequency Domain Scheduling (FDS) or beam-forming. Otherwise, The E-PDCCH transmission can be in multiple PRBs. In the latter case, if a NodeB has multiple transmitter antennas, the NodeB may transmit an E-PDCCH using antenna transmission diversity. Herein, an E-PDCCH transmitted in a single PRB is referred to as localized or non-interleaved, while an E-PDCCH transmitted in multiple PRBs is referred to as distributed or interleaved. Several aspects for an operation with interleaved E-PDCCHs or with non-interleaved E-PDCCHs need to be defined in order to provide a functional design. One aspect is the search process a UE performs to detect non-interleaved E-PDCCHs or to detect interleaved E-PDCCHs in a subframe. Another aspect is the detection of interleaved E-PDCCHs and of non-interleaved E-PDCCHs by a same UE in a same subframe. Another aspect is the treatment by a UE of PRBs configured to the UE for potential transmissions of interleaved E-PDCCHs or of non-interleaved E-PDCCHs when these PRBs are indicated to the UE for a PDSCH reception.
Therefore, there is a need to define a process for a UE to search for and decode non-interleaved E-PDCCHs and interleaved E-PDCCHs in a subframe.
There is also a need to define a method for a UE to decode both interleaved E-PDCCHs and non-interleaved E-PDCCHs in a same subframe.
There is also a need to determine rules for the treatment by a UE of PRBs configured to the UE for potential transmissions of interleaved E-PDCCHs or of non-interleaved E-PDCCHs when these PRBs are indicated to the UE for a PDSCH reception.